1. Field of the Invention
This invention relates generally to a method for treatment of skin by disrupting the targeted lesion in the skin. In particular, the invention relates to the generation of a microplasma in the skin to effect such disruption. Most particularly, the invention relates to the use of a pulsed beam of laser radiation to generate the microplasma.
2. Related Art
In many situations it is desired to have various vascular or pigmented lesions on the human skin treated or removed. Since the introduction of the selective photothermolysis model in the early 1980s, pulsed lasers have been widely used for treatment of vascular lesions such as port-wine hemangioma, telangiectasia, venous lakes, and spider and cherry angiomas, and pigmented lesions such as tattoos (decorative and traumatic), freckles, lentigos, epidermal pigmentation, and nevus.
Selective photothermolysis is discussed in detail in R. R. Anderson et al., "Selective Photothermolysis: Precise Micro Surgery by Selective Absorption of Pulsed Radiation," Science, Vol. 220, pp. 524-527, 1983, the disclosure of which is incorporated by reference herein. In selective photothermolysis, thermal disruption of targeted tissue (in the case of treatment of vascular lesions or targeted pigments in the case of treatment of pigmented lesions) is accomplished by impinging laser radiation on the tissue at a wavelength such that the radiation is preferentially absorbed by the targeted tissue over surrounding (collateral) tissue within an illuminated area. The absorption of the laser radiation by various chromophores in the targeted tissue causes the targeted tissue to rapidly heat up. The rapid heating causes a pressure build-up in the targeted tissue so that the targeted tissue is ablated once the laser fluence is above a threshold value. This process is often called "thermal ablation." Thermal ablation requires the use of laser radiation at an appropriate wavelength so that the laser radiation is strongly absorbed by the targeted tissue. Damage to the collateral tissue can be reduced by using a laser pulse with a duration shorter than the thermal relaxation time of the heated target tissue in order to achieve localized heating. However, the pulse must be sufficiently long so that the targeted tissue is heated sufficiently for thermal ablation to occur.
The wavelength selectivity and pulse duration requirements of selective photothermolysis can be readily met by pulsed dye lasers. For pulsed dye lasers, the wavelength of the laser radiation is determined by the type of dye used as a gain medium, and by a wavelength tuning element (e.g., a birefringent filter, an etalon or a prism) positioned in the laser cavity. The wavelength can be tuned almost continuously in the visible range from 450 nm to 650 nm. The typical pulse duration of pulsed dye lasers ranges from less than 1 microsecond to a few hundred microseconds.
A large focal spot (which may be located on or in the human skin), i.e., a focal spot having a diameter larger than 1 millimeter, has previously been employed in pulsed laser treatments. The practice of using a large focal spot is consistent with selective photothermolysis and has the benefits of reducing surgery time (since relatively fewer spots must be irradiated for a given treatment area) and reducing scattering loss as the laser beam passes through the dermal layer of the skin. However, a large focal spot requires a large pulse energy (typically several joules) for efficient ablation of target tissue. The use of large pulse energy undesirably results in excessive collateral tissue damage. Large pulse energy also necessitates the use of a large laser system which needs to be pumped by a flashlamp. Such a laser system is costly to manufacture and maintain, has low reliability due to the short lifetime of the flashlamp, is undesirably large and has low energy efficiency.
Therefore, although the use of flashlamp pumped, pulsed dye lasers has made the treatment of vascular lesions possible without significant damage to the skin surface due to the different absorption that is smaller at skin surface than that at targeted tissue, further improvements are desirable, such as reduced collateral tissue damage (resulting in shorter recovery time of a patient after a surgical procedure), increased reliability (i.e., longer lifetime and lower internal heat generation of the pumping system), and reduced cost to manufacture and maintain the laser.
In recent years, the use of Q-switched lasers for the treatment of pigmented lesions has become more popular due to the very short (i.e., nanosecond) pulses producible with such lasers. For typical Q-switched lasers, a solid state gain medium is used to generate a pulse train, each pulse having a duration between 10 and 100 nanoseconds. The pulses are typically generated at a pulse repetition rate up to several hertz. The laser-tissue interaction resulting from nanosecond laser pulses has been believed to be governed by the selective photothermolysis model. Consequently, the principles governing the design of pulsed dye lasers have been employed in the design of Q-switched lasers for use in dermatology and plastic surgery.
However, difficulties have been encountered with this approach. Due to the small number of available solid state gain media with reasonable lasing efficiency, the wavelength adjustability of Q-switched lasers has been very limited. To disrupt various types of tissue or pigment by thermal ablation with a single laser necessitates that the laser be capable of operating at each of the different wavelengths at which the various tissues and pigments are strongly absorbing. The limited wavelength adjustability of Q-switched lasers precludes use of a single Q-switched laser for treatment of all types of tissue and pigment. Although various nonlinear optical devices can be included in a Q-switched laser system to vary the wavelength of the laser output, the complexity of these devices and the loss of pulse energy as the laser radiation passes through the device has made laser systems including such nonlinear optical devices unreliable, inefficient and expensive.
A large focal spot has also been used in previous Q-switched laser treatments. As in the case of pulsed dye lasers, this requires large pulse energy and, therefore, a flashlamp pumped, Q-switched laser system. For nanosecond laser pulses, such as are used in Q-switched laser systems, it has been shown experimentally that the collateral tissue damage in the neighboring area of the ablation site is primarily determined by the pulse energy. Therefore, the use of large focal spots when using a Q-switched laser system for dermatology and plastic surgery could give rise to large amounts of collateral tissue damage resulting from the large pulse energy.
Significant technological advances have recently been made in the design and use of diode-laser pumped, solid state lasers, including Q-switched lasers. These new solid state lasers have the advantages of high energy efficiency, superior beam quality, compact size, and low maintenance cost. For example, the energy efficiency, which is defined as the average optical output power of the laser divided by the electrical input power to the laser, of a typical flashlamp pumped, pulsed laser is approximately 0.1% or less. In contrast, the energy efficiency of a diode-laser pumped, Q-switched laser can be as high as 10%. Moreover, the lifetime of the diode-laser is approximately 20 to 30 times longer than that of the flashlamp. At the present time, however, only diode-laser pumped, Q-switched lasers with a small pulse energy (on the order of magnitude of 10 millijoules) can be manufactured at a cost less than or comparable to the flashlamp pumped lasers. For the large focal spots typically used, these low energy lasers cannot efficiently ablate tissue.
The use of Q-switched lasers with small pulse energy is desirable in order to reduce excessive collateral tissue damage (i.e., patient recovery time) and to take advantage of the recent developments in diode-laser pumping technology that have enabled production of a reliable and low cost Q-switched laser system. However, in order to use low energy Q-switched lasers, new approaches for using Q-switched lasers in the treatment of vascular and pigmented lesions in human skin, other than selective photothermolysis, have to be introduced.